Japanese Patent Publication No. 3115456 (hereinafter referred to as “prior art 1”) discloses a known laser beam welding process of welding two plates, each having a base metal covered with a layer of a metal whose melting point is lower than the melting point of the base metal, overlapping each other.
According to the disclosed laser beam welding process, two steel sheets plated with zinc, i.e., two galvanized steel sheets, each having a steel sheet as a base metal covered with a layer of zinc whose melting point is lower than the melting point of the base metal, overlap each other, and supported with their ends unconstrained. Only one of the galvanized steel sheets to which the laser beam is applied is melted in advance by the laser beam at a position which is displaced 1 mm through 15 mm from the center of the welded region of the overlapping steel sheets toward the constrained region thereof. The end of the galvanized steel sheet which is irradiated with the laser beam is deformed, creating a gap between the galvanized steel sheets. Thereafter, the superposed galvanized steel sheets are welded to each other.
According to the prior art 1, in order to deform the end of the galvanized steel sheet, the laser beam is applied to melt the galvanized steel sheet, and the galvanized steel sheet is spaced from the other galvanized steel sheet upon solidification and shrinkage of the melted surface. Depending on how the galvanized steel sheet is melted, the amount of deformation of the end of the galvanized steel sheet varies. For example, the amount of deformation of the end of the galvanized steel sheet may be reduced even if the amount by which the galvanized steel sheet is melted increases.
Specifically, as shown in FIG. 15 of the accompanying drawings, a laser beam L was applied to a galvanized steel sheet 1 having a thickness t, and the depth D of a melted pool 2 produced in the galvanized steel sheet 1 by the laser beam L and the amount K by which the galvanized steel sheet 1 is deformed at a position spaced 10 mm from the position where the laser beam L is applied were detected for different samples. The thickness t was 0.7 mm and 2.0 mm, and the laser beam L was moved at speeds (machining speeds) of 5 m/min. and 2 m/min. At various laser output settings (w), the following results were obtained:
TABLE 1MachiningThicknessspeedOutputDH(mm)(m/min)(w)(mm)(mm)0.753000.1060.1300.756000.1990.2200.759000.2350.2700.7512000.3320.2740.7515000.4440.0972.028000.3240.0592.0210000.6420.1942.0212000.9760.9702.0215001.5970.112
It can be seen from Table 1 that the galvanized steel sheet 1 cannot be deformed as desired simply by increasing the laser output setting to increase the amount by which the galvanized steel sheet is melted. Therefore, a zinc gas vaporized when two galvanized steel sheets 1 are welded is not well discharged out, resulting in a welding failure due to blowhole, etc.
According to the conventional laser beam welding process disclosed in the prior art 1, it is necessary to apply a first laser beam to deform the end of one of the galvanized steel sheets and also to apply a second laser beam to weld the overlapping regions of the galvanized steel sheets, independently of each other. Therefore, the entire laser beam welding process is considerably complex and has low productivity.
For joining the ends of three or more overlapping sheets with a laser beam, it is customary to apply the laser beam in the direction in which the sheets overlap each other for welding the ends of the overlapping sheets. If the overlapping sheets to be welded are thick, then a large amount of heat needs to be applied to melt the sheets, resulting in a large thermal strain which makes it impossible to perform a desired laser beam welding process.
Other welding processes for welding overlapping sheets are disclosed in Japanese Laid-Open Patent Publication No. 7-16775 (hereinafter referred to as “prior art 2”) and Japanese Laid-Open Patent Publication No. 9-206969 (hereinafter referred to as “prior art 3”).
According to the prior art 2, as shown in FIG. 16 of the accompanying drawings, of three overlapping flanges 1a, 2a, 3a are to be welded, the flange 1a at one end in the overlapping direction, i.e., the direction in which the flanges 1a, 2a, 3a overlap each other, and the flange 2a adjacent to the flange 1a are irradiated with a high-density laser beam which is applied in the overlapping direction. The flanges 1a, 2a are welded together by the applied high-density laser beam, with a bead 4a formed across the flanges 1a, 2a in the overlapping direction.
Then, the flanges 2a, 3a are welded to each other by a laser beam which is applied to the edges thereof in a direction perpendicular to the overlapping direction parallel to the interfaces of the flanges 2a, 3a. A bead 5a is formed in the flanges 2a, 3a along the interfaces thereof in the direction perpendicular to the overlapping direction.
According to the prior art 3, as shown in FIG. 17 of the accompanying drawings, three sheets including an outer panel 6a, an inner panel 6b, and a reinforcement 6c have overlapping ends to be welded into a joint 8a by a laser beam welding machine 7a. The laser beam welding machine 7a applies a laser beam to the edge of the joint 8a while moving in a zigzag pattern over the edge of the joint 8a, thus heating the edge of the joint 8a and forming a zigzag welded seam 9a. 
With the prior art 2, when the three flanges 1a, 2a, 3a are to be welded together, the first laser beam is applied in the overlapping direction to weld the flanges 1a, 2a to each other, and the second laser beam is applied in the direction perpendicular to the overlapping direction to weld the flanges 2a, 3a to each other. The welding process according to the prior art 2 involves a number of welding steps and is low in productivity the three sheets need to be irradiated with the two laser beams applied in the different directions.
With the prior art 3, the welding system is complex in structure and control as the laser beam welding machine 7a has to be moved in the zigzag pattern, and may possibly fail to weld the interfaces of overlapping sheets having different thicknesses and materials.